Add DD_AGENT_FEATURE_POLLING_ENABLED setting

tracer/src/Datadog.Trace/Agent/DiscoveryService/DiscoveryService.cs

@@ -17,6 +17,9 @@
 
 namespace Datadog.Trace.Agent.DiscoveryService
 {
+    /// <summary>
+    /// Queries the Datadog Agent and discovers which version we are running against and which endpoints it supports.
+    /// </summary>
     internal class DiscoveryService : IDiscoveryService
     {
         private const string SupportedDebuggerEndpoint = "debugger/v1/input";

tracer/src/Datadog.Trace/Agent/DiscoveryService/IDiscoveryService.cs

@@ -11,7 +11,7 @@
 namespace Datadog.Trace.Agent.DiscoveryService
 {
     /// <summary>
-    /// Queries datadog-agent and discovers which version we are running against and what endpoints it supports.
+    /// Queries the Datadog Agent and discovers which version we are running against and which endpoints it supports.
     /// </summary>
     internal interface IDiscoveryService
     {

tracer/src/Datadog.Trace/Ci/TestOptimizationTracerManagerFactory.cs

@@ -107,7 +107,7 @@ protected override IAgentWriter GetAgentWriter(TracerSettings settings, IDogStat
             return new ApmAgentWriter(settings, updateSampleRates, discoveryService, traceBufferSize);
         }
 
-        protected override IDiscoveryService GetDiscoveryService(TracerSettings settings)
+        internal override IDiscoveryService GetDiscoveryService(TracerSettings settings)
             => _testOptimizationTracerManagement.DiscoveryService;
     }
 }

tracer/src/Datadog.Trace/Configuration/ConfigurationKeys.cs

@@ -585,6 +585,13 @@ internal static partial class ConfigurationKeys
         /// </summary>
         public const string ApplicationMonitoringConfigFileEnabled = "DD_APPLICATION_MONITORING_CONFIG_FILE_ENABLED";
 
+        /// <summary>
+        /// Configuration key to disable polling the /info endpoint in the trace agent for feature discovery.
+        /// Default value is true (polling enabled).
+        /// </summary>
+        /// <seealso cref="TracerSettings.AgentFeaturePollingEnabled"/>
+        public const string AgentFeaturePollingEnabled = "DD_AGENT_FEATURE_POLLING_ENABLED";
+
         /// <summary>
         /// String constants for CI Visibility configuration keys.
         /// </summary>

tracer/src/Datadog.Trace/Configuration/TracerSettings.cs

@@ -457,6 +457,10 @@ not null when string.Equals(value, "otlp", StringComparison.OrdinalIgnoreCase) =
                                              .WithKeys(ConfigurationKeys.AzureServiceBusBatchLinksEnabled)
                                              .AsBool(defaultValue: true);
 
+            AgentFeaturePollingEnabled = config
+                                        .WithKeys(ConfigurationKeys.AgentFeaturePollingEnabled)
+                                        .AsBool(defaultValue: true);
+
             DelayWcfInstrumentationEnabled = config
                                             .WithKeys(ConfigurationKeys.FeatureFlags.DelayWcfInstrumentationEnabled)
                                             .AsBool(defaultValue: true);
@@ -990,6 +994,15 @@ not null when string.Equals(value, "otlp", StringComparison.OrdinalIgnoreCase) =
         /// <seealso cref="ConfigurationKeys.AzureServiceBusBatchLinksEnabled"/>
         public bool AzureServiceBusBatchLinksEnabled { get; }
 
+        /// <summary>
+        /// Gets a value indicating whether the agent discovery service is enabled.
+        /// When disabled, the tracer will not query the agent for available endpoints.
+        /// This is useful in environments where the discovery endpoint is not available (e.g., Azure Functions with Rust agent).
+        /// Default value is true (discovery service enabled).
+        /// </summary>
+        /// <seealso cref="ConfigurationKeys.AgentFeaturePollingEnabled"/>
+        public bool AgentFeaturePollingEnabled { get; }
+
         /// <summary>
         /// Gets a value indicating whether to enable the updated WCF instrumentation that delays execution
         /// until later in the WCF pipeline when the WCF server exception handling is established.

tracer/src/Datadog.Trace/TracerManagerFactory.cs

@@ -458,9 +458,6 @@ private static string GetUrl(TracerSettings settings)
             }
         }
 
-        protected virtual IDiscoveryService GetDiscoveryService(TracerSettings settings)
-            => DiscoveryService.Create(settings.Exporter);
-
         internal static IDogStatsd CreateDogStatsdClient(TracerSettings settings, string serviceName, List<string> constantTags, string prefix = null, TimeSpan? telemtryFlushInterval = null)
         {
             try
@@ -507,6 +504,11 @@ internal static IDogStatsd CreateDogStatsdClient(TracerSettings settings, string
             }
         }
 
+        internal virtual IDiscoveryService GetDiscoveryService(TracerSettings settings)
+            => settings.AgentFeaturePollingEnabled ?
+                   DiscoveryService.Create(settings.Exporter) :
+                   NullDiscoveryService.Instance;
+
         private static IDogStatsd CreateDogStatsdClient(TracerSettings settings, string serviceName)
         {
             var customTagCount = settings.GlobalTags.Count;

tracer/test/Datadog.Trace.Tests/Telemetry/config_norm_rules.json

@@ -309,6 +309,7 @@
   "dbm_propagation_mode": "dbm_propagation_mode",
   "trace.remove_root_span_laravel_queue": "trace_remove_root_span_laravel_queue_enabled",
   "trace.remove_autoinstrumentation_orphans": "trace_remove_auto_instrumentation_orphans_enabled",
+  "DD_AGENT_FEATURE_POLLING_ENABLED": "agent_feature_polling_enabled",
   "DD_TRACE_AWS_ADD_SPAN_POINTERS": "trace_aws_add_span_pointers",
   "DD_TRACE_CONFIG_FILE": "trace_config_file",
   "DD_DOTNET_TRACER_CONFIG_FILE": "trace_config_file",

tracer/test/Datadog.Trace.Tests/TracerManagerFactoryTests.cs

@@ -103,6 +103,29 @@ public void RemoteConfigIsDisabledInAzureAppServices()
         _manager.TracerFlareManager.Should().BeOfType<NullTracerFlareManager>();
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void DiscoveryServiceCanBeDisabled(bool enabled)
+    {
+        var source = CreateConfigurationSource((ConfigurationKeys.AgentFeaturePollingEnabled, enabled.ToString()));
+        var settings = new TracerSettings(source);
+
+        settings.AgentFeaturePollingEnabled.Should().Be(enabled);
+
+        var factory = new TracerManagerFactory();
+        var discoveryService = factory.GetDiscoveryService(settings);
+
+        if (enabled)
+        {
+            discoveryService.Should().BeOfType<DiscoveryService>();
+        }
+        else
+        {
+            discoveryService.Should().BeSameAs(NullDiscoveryService.Instance);
+        }
+    }
+
     private static TracerManager CreateTracerManager(TracerSettings settings)
     {
         return new TracerManagerFactory().CreateTracerManager(